1. Field of the Invention
This invention relates to viscosity stable chloroprene-sulfur copolymers.
2. Prior Art
"Sulfur modified" neoprenes (so called, but such compositions are in fact copolymers of chloroprene and sulfur) have had and continue to have wide commercial success. The copolymers are prepared by polymerizing chloroprene in an aqueous emulsion in the presence of sulfur as widely described in the art, for example, Whitby, "Synthetic Rubber", 1954, page 770. The chloroprene is polymerized with sulfide linkages in the polymer chains. The product is a high molecular weight, high viscosity polymer, which would not be tractable in ordinary rubber milling and processing equipment.
Generally the product rubber is subjected to a procedure to cause scission of the long molecular chains at the sulfide linkages to reduce the molecular weight of the rubber and produce rubber of the desired viscosity. The process of chain scission has been called "plasticizing" at times, however, it is believed the more appropriate term is peptization.
Tetraethyl thiuram disulfide (TETDS) is the principal peptizing agent and it causes the desired scission of the polymer chains at the sulfide linkages.
A small amount of the TETDS remains in the rubber after peptization. This is advantageous, since it functions to further peptize the rubber during mixing operations, and also improves the processing safety of the rubber.
Unfortunately, the peptization of the rubber is not confined to the mixing step and the reaction continues during storage, and eventually the polymer will become unsuitable for use. Even when it is still useful, the viscosity variation in the same batch of rubber, makes manufacturing use of the rubber difficult. Even costly refrigeration during shipment and storage of the rubber does not overcome this problem, although the process may be slowed.
A wide variety of stabilizers has been added to the rubber to inhibit this decline. For example, U.S. Pat. No. 3,397,173 to Collette et al, solved this problem by employing dialkyl xanthogen disulfide (a chain transfer agent which performs the opposite function of elemental sulfur in that it causes a termination of polymer chains) in the chloroprene-sulfur polymerization. Nonetheless, the patentee also further stabilized the viscosity by adding secondary amines to the latex before isolation of the rubber. Unfortunately, the continued use of this system is questionable, since the dialkyl xanthogen disulfides have been found to constitute a serious health hazard to workers.
Hence, the desirable chloroprene-sulfur copolymer, which is available, is the older type described by Whitby, supra, with its associated problems described by Collette et al. The procedure disclosed by Collette et al employed secondary amine antioxidants with the plasticized rubber, however, many of these compounds were found to cause staining.
The present invention provides a method of providing viscosity stable chloroprene-sulfur copolymer, which requires no further antioxidant treatment of the rubber after polymerization. Another advantage of the present invention is that a simplier "sulfur modified" polychloroprene preparative method is provided.
These and other advantages will be apparent from the following: